1. Field of the Invention
The present invention relates to a mechanism for driving plungers of metal molds in a bottle-making machine. More specifically, the invention relates to a mechanism for driving plungers in order to mold a glass material thrown into the metal molds, in the sections of a bottle-making machine, into desired shapes.
2. Prior Art
A bottle-making machine usually has 6 to 12 sections, and in each section are arranged 1 to 4 metal molds (coarse molds) depending upon the size of the metal molds (due to limitation in space). A glass material (gob) is thrown from a glass material feeder (gob feeder) into the metal molds in each of the sections, successively, and the glass material is molded into articles (parisons) of a desired shape by the operation of a plunger provided for each of the metal molds. So far, an air cylinder has been provided for the plunger of each of the metal molds, and is advanced with a predetermined torque to effect the molding in each of the metal molds.
Though the air cylinder has heretofore been used as a mechanism for driving the plunger as described above, the output cost of the air cylinder is never good and is, usually, lower by 20 to 50% than the output cost of an electric motor. So far, furthermore, independent mechanisms are necessary for driving the plungers of the metal molds in each of the sections. As the number of the metal molds increases from two to three or four, the apparatus becomes complex and the control system becomes complex, too.
It has already been proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-142853 to actuate the plungers in the bottle-making machine of this kind by using an electric motor (servo motor). According to this prior art, a servo motor is arranged in each metal mold to actuate the plunger, and each metal mold is provided with a mechanism for detecting the position of the plunger to effect the molding while detecting the position at where the pressure exerted by the plunger terminates.
According to this prior art, however, each plunger in each metal mold is provided with a servo motor, which is a drive mechanism in place of the conventional air cylinder, without contributing to simplifying the apparatus or the control operation therefor.
According to this prior art, furthermore, the molding is effected while detecting a position where the pressure exerted by the plunger terminates. Such a control operation may be effective when the amounts of the glass material in the metal molds are strictly equal to each other. In practice, however, the glass material (gob) is supplied in varying amounts from the glass material feeder (gob feeder) to the metal molds. Depending upon the controlled positions of the plungers, therefore, the molding becomes defective due to lack of pressure when the amount of the material is small, or the molding becomes defective such as mold opening due to excess pressure when the amount of the material is great.